For example, if you are multiplying 19 x 21, do this:
square the middle number (multiply it by itself) 20 x 20 = 400
subtract 1, there's your answer: 399
Practice: 29 x 31
square the middle number (30 x 30) = 900
subtract 1, there's your answer: 899
It works with single digits, try 5 x 7
try 4 x 6
but it's really handy for the bigger numbers.
Try 99 x 101
100 x 100 is 10,000, so the answer is 9,999
OK, ready for the slightly more challenging ones?
Try 23 x 25.....try it on your own before scrolling to the answer....
24 x 24 is (24 x 20 ( 480) + 4 x 24 (96) = 576) - 1 = 575
You might be (rightfully) asking "How is this easier than just multiplying 23 x 25?"
A: It's just another tool in your toolbox of strategies, giving you more options to choose from. You'll see it's easier to use this method if the middle number is easy to square.